Do know if it would be ok to do it the other way round? Buy the upgrade for one computer but also use it on the computer eligible for the up-to date disk?
You could certainly use an upgrade disk on any Mac using Leopard. The up-to-date disk will almost certainly just be an upgrade disk at a lower price. Going on past experience anyway (iLife up-to-date and a few others) It's generally not worth the hassle for Apple to manufacture multiple different builds of an OS